1. Field of the Invention
This invention is a further development in the field of alkali metal leak detection, particularly with respect to liquid sodium heat transfer systems for nuclear reactors.
2. Description of the Prior Art
A number of techniques have been developed to detect a sodium leak in the coolant system of a nuclear reactor. The most common technique at the present time involves the placement of a pair of electrodes, separated from one another by a suitable gap, in proximity to the sodium containment structure. When a sodium leak occurs, sodium will condense or flow onto the electrodes and will eventually cause a short circuit between the electrodes. This short circuit will activate an alarm or recording device. There are two major deficiencies inherent in this technique. First, the path of the condensing or flowing sodium after it leaks from its containment structure is unpredictable, and there is no assurance that the leaking sodium will ever come into contact with the electrodes to produce a short circuit. Second, the electrodes are sensitive to short-circuiting by materials other than leaking sodium, which presents the possibility of false leak alarms.
Another technique involves the impingement of light from a sodium vapor lamp onto a photoelectric cell in the vicinity of the sodium containment structure. When sodium leaks from the containment structure, the sodium atoms "cloud" the region between the sodium vapor lamp and the photoelectric cell. The sodium atoms in the cloud absorb light from the lamp, thereby reducing the intensity of the light impinging upon the photoelectric cell. The reduction in intensity of the light impinging on the photoelectric cell can be sensed to activate a leak alarm or recorder. However, at least one of the deficiencies inherent in the short-circuiting technique described above is likewise inherent in this atomic absorption technique. The leaking sodium can condense on surfaces external to the containment structure and never travel to the region between the lamp and the photoelectric cell. The unpredictability of the path of the leaking sodium renders both the short circuiting technique and the atomic absorption technique ineffective in assuring rapid detection of a sodium leak.
Another technique involves confinement of the atmosphere in the region proximate to the external surface of the sodium containment structure, and the forcing of this confined atmosphere into water solution at regular intervals. Any sodium which has leaked into this region, whether it remains in pure form or has oxidized, will cause the water to become alkaline. The pH of the water can be monitored, and a change in pH can cause the activation of an alarm or recording device. The main problem with this technique, aside from its requirement for extensive ancillary equipment, is that it is not sufficiently sensitive to detect minute sodium leaks. Minute changes in pH of a large volume of water are difficult to detect. Furthermore, leaking sodium can condense on structural surfaces without mixing in the confined atmosphere that will pass into water solution for pH testing.
Still another technique involves monitoring the oxygen gas content of a confined atmosphere in the vicinity of the sodium containment structure. Leaking sodium will react chemically with the oxygen gas, and thereby reduce the amount of oxygen gas in the atmosphere. The main problem with this technique is that it is not sufficiently sensitive to detect minute sodium leaks. Extremely small changes in the oxygen gas content of a large volume of monitored atmosphere are difficult to detect.
Existing sodium leak detection techniques are not amenable to calibration and checking for positive reliability without causing sodium to enter the detection system. A controlled sodium leak is difficult to achieve, particularly because of the unpredictability of the path the leaking sodium will take. Since it is difficult to remove all traces of sodium from the detection system after an efficiency test of the system, it is very undesirable to test the effectiveness of present sodium leak detection systems.